1. Field of the Invention
The present invention relates to integrated circuits, and particularly, to a transmission power amplifier suitable for use in a mobile phone having a switch circuit for switching operation mode and built with a multi-chip module (MCM) technology.
2. Description of Related Art
As mobile phones become more and more functional, the amount of information transmission is ever increasing, starting with mere voice communication to internet connection and image transmission. Thus, a variety of digital modulation techniques capable of high-speed, large capacity transmission are used in wireless transfer. A transmission power amplifier used in a mobile phone and so on using the digital modulation technique is required to have a low error rate in digital data and high power efficiency. To meet both requirements, some mobile phones employ both of a Enhanced Data GSM Environment (EDGE) system for linear operation with a low digital data error rate and a Global System for Mobile communication (GSM) system for operation with high power efficiency. Such mobile phones have a control terminal for switching the EDGE system and the GSM system to switch load resistance of the amplifier, thereby changing power efficiency. Further, the power amplifier is packaged in MCM where a matching circuit constituted by an active element having a heterojunction bipolar transistor (HBT) structure and a passive element switching the functions of the active element are formed on a principal surface of a semi-insulating compound semiconductor substrate. Thus, higher function, smaller-size, and lower-cost products progressively come into practical use in a several GHz range.
The operation of a conventional transmission power amplifier for a mobile phone is explained below with reference to FIG. 4. FIG. 4 is a circuit diagram to explain an example of a conventional transmission power amplifier used in a mobile phone and so on. The transmission power amplifier of FIG. 4 is a three-stage power amplifier composed of three HBTs Q1, Q2, and Q3. This circuit switches between high power efficiency operation mode (GSM) and linear operation mode (EDGE) by a V-mode terminal voltage. Generally, the linear operation mode performs backoff to a power level of 10 to 15 dB lower from the saturation state of the amplifier. As shown in FIG. 4, a first stage amplifier HBT Q1, a second stage amplifier HBT Q2, and a final stage amplifier HBT Q3 are configured as a grounded emitter amplifier circuit in which input or collector output of the previous stage is inputted to its base. The base of the HBT Q1 is connected to an input terminal IN, through which an RF signal is inputted. The collector of the HBT Q3 is connected to an output terminal OUT. Capacitors C1, C2, C3, and C4 for direct current cutting are connected in series between the input terminal IN, the HBTs Q1, Q2, Q3, and the output terminal OUT, respectively. Bias voltages Vb1, Vb2, Vb3, Vcc1, Vcc2, and Vcc3 are supplied to the base and collector of the HBTs Q1, Q2, and Q3, respectively.
The circuit of FIG. 4 also includes a matching circuit for switching the GSM mode and the EDGE mode. The matching circuit is constituted by connecting a diode D1 to the base of the first stage amplifier HBT Q1 and to the base of the second stage amplifier HBT Q2 via a capacitor C5 and a capacitor C6, respectively, connected in series, and connecting a resistor R3 in parallel with the cathode terminal of the diode D1. Further, a V-mode terminal C-IN is connected the anode of the diode D1 so as to turn on or off the diode D1 by the voltage inputted to the V-mode terminal C-IN, thereby switching the GSM mode and the EDGE mode.
In the transmission power amplifier with the above matching circuit connected thereto, when setting the EDGE mode, if a voltage higher than a forward voltage of the diode D1 is inputted to the V-mode terminal C-IN to turn on the diode D1, the RF signal from the input terminal IN is attenuated through the diode D1 and the resistor R3, decreasing the effective gain of the first stage amplifier HBT Q1. On the other hand, if a voltage lower than the forward voltage of the diode D1 is inputted to the V-mode terminal C-IN to turn off the diode D1, the RF signal does not pass through the diode D1, thereby increasing the effective gain of the first stage amplifier HBT Q1. Japanese Unexamined Patent Application Publication No. 07-202613, for example, describes the circuit which has the control terminal C-IN in addition to the input terminal IN so as to adjust the gain. Japanese Unexamined Patent Application Publication No. 58-051609 describes another circuit which automatically controls the gain.
It has now been discovered that current consumption increases in the conventional power amplifier shown in FIG. 4 since it requires a current flow path from the V-mode terminal C-IN for mode switching control to the resistor R3 through the diode D1. Further, power efficiency decreases since the gain of the first stage amplifier HBT Q1 decreases as the RF signal from the input terminal IN is attenuated. Furthermore, the circuit of FIG. 4 and the circuits described in Japanese Unexamined Patent Application Publications No. 07-202613 and 58-051609 require an additional terminal to control gain and a circuit to generate a V-mode voltage in addition to the power amplifier circuit. This complicates the circuit for the baseband control of a mobile phone set.